Curiosity rover's descent to Mars – the story so far

In August 2012, Nasa succeeded in landing the Curiosity rover safely on the surface of Mars. We look back to an incredible feat, talk to the team involved and consider the ramifications of Curiosity's findings to date

Flight director Keith Comeaux, right, celebrates with engineer Martin Greco at Nasa’s Jet Propulsion Laboratory on 5 August 2012, following the successful descent of the Curiosity rover to the surface of Mars. Photograph: Damian Dovarganes/AP

Nestled below the foothills of the San Gabriel mountains, the Jet Propulsion Laboratory outside Pasadena has a surprisingly low-tech feel. For more than 40 years, space missions to the planets have been controlled from its operations rooms, yet the place is still striking for its bucolic charm. Mule deer crisscross its paths, pausing only to nibble plants, while its buildings, erected during the heyday of the US space programme, now have a settled, slightly worn aspect.

For a Californian campus, it is all very laid-back – on most occasions. On 5 August last year, however, JPL was far from being a peaceful place. Indeed, it was in tumult. Thousands of anxious scientists and engineers had gathered to track the fate of the Curiosity Mars rover, the most sophisticated interplanetary probe in history, which was now about to plunge toward the surface of the Red Planet after a journey of 354 million miles.

Given that Nasa itself was advertising Curiosity's descent through the Martian atmosphere as "seven minutes of terror", the probe was playing havoc with the emotions of the men and women who had devoted years to its construction. Lucky beards were left unshaved and favourite T-shirts worn. Peanuts – considered to be a particular bringer of good fortune at previous JPL landings – were gulped in handfuls. One engineer, Bobak Ferdowsi, even sported a special mohawk haircut with star shapes shaved into it.

Flight director Bobak Ferdowsi, whose Twitter account gained 20,000 followers, not to mention marriage proposals and an invitation to President Obama’s re-election parade, when he displayed the special star-studded mohawk he had created to mark the Curiosity landing.

All missions have their nerve-racking moments, of course. Curiosity's case was special, however, because of the vast numbers of people involved. A decade earlier, Nasa had decided to ramp up its efforts to study Mars and construct the grandaddy of all mobile laboratories. The one-tonne craft would be nuclear-powered and be fitted with a plethora of instruments constructed to answer one simple question: had Mars ever possessed conditions that could support life? More than 7,000 scientists and engineers – an unprecedented number – were called in to help overcome the challenges in designing, building and operating Curiosity.

Just selecting a site for the giant rover's landing had proved a major headache, for example. More than 60 places were considered by mission staff, a list from which Gale Crater – near the Martian equator – was eventually selected. It is deep, boulder-strewn and has strong drafts of air sweeping up from its floor. "That made it a difficult place to land in," admits Bill Dietrich, a member of the Curiosity science team. "On the other hand it is geologically fascinating."

At the centre of Gale, Mt Sharp rises from the crater floor to a height of 16,000ft. Satellite photographs show it is made of layers that seem to delineate Mars's entire geological history. Set down at its base, Curiosity could trundle up its flanks to study how the Red Planet had changed over the past 4.5 billion years. The trick would be getting there.

In the past, US space engineers had adopted the bouncy castle approach to putting their rovers on Mars. A robot vehicle was secured inside a bag which was inflated during descent so that it simply bounced over the surface until it came to a standstill. For Curiosity, this was not an option. Five times bigger than its predecessors, it would have required an airbag so big, the stresses of descent would have ripped its fabric apart. Similarly, landers that use legs were considered too unstable to settle on Mars.

It took a marathon brainstorming session of Curiosity scientists and engineers at the Jet Propulsion Laboratory in September 2003 to come up with a solution: a rover on a rope. According to the plan proposed by Adam Steltzner, leader of the probe's Entry Descent and Landing (EDL) team, a platform of rocket thrusters – subsequently dubbed the Sky Crane – would hover above the Martian surface before lowering Curiosity by cable. The system was accurate, manoeuvrable and could drop the rover precisely where scientists wanted it to go. The concept was also novel, untested and contained dozens of potential flaws, any one of which could doom the mission.

Not surprisingly many mission scientists were doubtful. "I remember when I was first shown the plan. I thought: they must be joking!" says Dietrich. Many of Nasa's bigwigs were also alarmed. The mission had a $2.5bn price-tag, after all. Steltzner – a former rock musician-turned-astrophysicist – was summoned to Washington by Mike Griffin, then the head of the agency, to justify his plan. Steltzner defended it with aplomb. For his part, Griffin announced he still thought the concept was crazy but "maybe just the right sort of crazy". The Sky Crane was approved and on 26 November 2011, the device – attached to Curiosity – was blasted into space. On 5 August 2012, it reached its target.

It takes between four and 21 minutes for radio signals from Earth to reach Mars, depending upon the planets' relative positions. Last August, the distance between the two was such that radio signals took around 14 minutes to reach Mars. Guiding a complex landing sequence from California was never a prospect for this reason. The whole process would, instead, be run by on-board computers whose performance would determine the fate of the device created by the thousands of nervy scientists and engineers who had gathered in rooms and halls round JPL. The future of their brainchild depended on a sequence of parachutes and rocket thrusters being deployed automatically and accurately to slow Curiosity from its atmosphere-entry speed of 13,000mph to a landing velocity of 1.7mph.

Steltzner – who had spent the previous decade working on Curiosity – gathered his Entry, Descent and Landing (EDL) team in the mission's control room. "I had spent so much time thinking about the ways in which it couldn't work, or wouldn't work, that the idea of feeling that it would work, of relaxing and trusting that it would work, felt like a dereliction of duty to me," he recalls. "I was rationally confident but emotionally terrified."

Each stage of the descent passed perfectly until the Sky Crane was deployed. Curiosity had only seconds before it reached the surface. The room was utterly silent until touchdown was confirmed and the place erupted to the sound of cheering, screaming, high-fiving EDL engineers. "I felt strangely numb, exhilarated, and slightly in disbelief," Steltzner remembers.

Charles Bolden, the new head of Nasa, was less circumspect. At a quickly convened press conference, he gloated about American ingenuity. No other nation had landed on the Red Planet but the US had now placed seven craft on its surface. Bolden's eulogy was interrupted by Steltzner's team, which had already reached the early stages of cathartic release. They gathered outside chanting, "E-D-L! E-D-L!" before bursting into the press room to attempt to conga their way though it. The bars of Pasadena did stellar business that night while Ferdowsi – with his star-studded mohawk haircut – became a celebrity, receiving 20,000 new Twitter followers, several marriage proposals and, subsequently, an invitation to join President Obama's re-election inaugural parade.

A self-portrait of the Mars rover Curiosity. Photograph: NASA/Getty Images

Over the next few days, the engineers and scientists who had built the myriad devices fitted to Curiosity reported, without exception, that each had been tested and was working perfectly. Like an athlete limbering up before a race, the rover's components were gently put through their paces before the six-wheeled Curiosity trundled off on its search for clues to the past habitability of Mars. It did not have to travel far.

"We could see from satellite photographs, that a fan of sedimentary material – like an alluvial delta produced by a river of water on Earth – appeared to have spread through the Gale Crater from one point on its northern rim and had passed very close to Curiosity's landing place," says Dietrich. "It was a perfect target." By December, the rover had travelled about a kilometre from its landing site and had reached the edge of the fan at a spot named Yellowknife Bay by mission scientists. Images from satellites showed the ground here retained its heat even at night, a strong sign that it contained clays and other sediments. Zapping some of the rocks with lasers also indicated the presence of gypsum, a mineral associated on Earth with the presence of water. It was time to drill.

Louise Jandura had led the team of 50 engineers who designed Curiosity's sampling machinery, which includes a drill and a robot arm that delivers quantities of powdered rock to detectors inside the rover. "Before Curiosity made its first hole on Mars, we had carried out more than 1,200 tests on Earth on 20 different types of rock," she recalls. "We went to a lot of pains to get it right."

The day of Curiosity's first drill – on 8 February at Yellowknife Bay – was still a nerve-racking experience, nevertheless. Jandura and her team had been asked to design and build machinery that could produce samples of finely powdered Martian soil – the equivalent in volume of half a crushed aspirin – from a maximum depth of 6.5cm and to deliver those samples to Curiosity's chemical analysers. "We watched the drill go in. Later we saw the powdered rock in the scoop that carries samples to the detectors. The whole apparatus worked perfectly. It was so, so satisfying after eight years of effort. We were hooting and hollering and ended up partying in a local pub that night."

There would be more to celebrate than engineering success, however. The rock sample was divided and placed in two of Curiosity's suites of detection equipment. One was an x-ray diffraction device that measures the abundance of elements in rocks and soils. The second was Sam – Sample Analysis at Mars – which exploits spectrographic techniques to analyse a sample's constituent chemicals. Remarkably, the two instruments – despite exploiting very different scientific procedures – produced identical results.

"We got the same result: the rock drilled from Yellowknife Bay contained clays that could only have been created through sustained reactions between water and rocks," says project scientist Ashwin Vasavada. Photographs taken by Curiosity's cameras also revealed images of rounded pebbles like those moulded by streams on Earth, while other analysis showed the soil was neither too alkaline nor too acidic when those clays were created. "The water must have been drinkable then," says Vasavada.

Billions of years ago, fresh water had poured down the northern rim of Gale Crater and flowed over the rocks of Yellowknife Bay. More to the point, those rocks had been bathed in water for long enough to sustain reactions that produced the clays that the rover had detected. Curiosity had found the first habitable place on another world.

The clays detected by Curiosity have since been shown to be of a type called smectites. Typically these form in the sediments of lakes. "It is therefore quite likely that the entire Gale Crater was once flooded, possibly to a depth of more than a kilometre," adds Dietrich, who is also head of planetary sciences at the University of California, Berkeley. A place of grim desiccation today, it appears this part of Mars positively gurgled with water billions of years ago.

Landforms on Mars, an image taken by the High Resolution Imaging Science Experiment (HiRISE) flying onboard the Mars Reconnaissance Orbiter mission. Photograph: Nasa

For the project's engineers and scientists, the discovery was particularly satisfying because it revealed the power of Curiosity and its array of instruments. Tens of thousands of tests have now been carried out on Mars by the rover, threatening to engulf Nasa and its scientists in data. Powered by a tiny nuclear generator, Curiosity can be run day and night. Previous rovers have relied on solar panels, which were poor power providers and could only operate effectively during the day. Only a relatively small number of instruments could be fitted and operated as a result.

"In the past, experiments that produced ambiguous results on Mars could not be repeated or compared with tests carried out by other instruments," says Pan Concord, a deputy principal investigator for the Sample Analysis at Mars device. "Here, we can go on and on until we are absolutely sure of our results. That is what we did at Yellowknife. It was wonderful. And I am sure we will do it again."

However, it is the wider implications of Curiosity's discovery at Yellowknife Bay that excites Michael Meyer, lead scientist for Nasa's Mars Exploration Programme. "We sent Curiosity to Mars to find out if the planet had the potential to sustain life – and that it is exactly what it has done." Fresh water, the matrix of life, was once abundant on Mars until an unknown planet-wide catastrophe engulfed it and turned it into an arid, lost world.

Finding the nature of that catastrophe will be a future task for Curiosity. Most scientists suspect Mars's low mass and its lack of a magnetic field may have doomed it. On Earth, our relatively strong gravitational field plus our relatively intense magnetic field protect our atmosphere from battering by the solar wind, a constant stream of particles that pours from the sun. Without such fields, Mars could not hold on to its atmosphere or its water, which were swept into space. Earth remained blue and watery while Mars turned to dust. Over the next few years, Curiosity's Sam detector will sniff out delicate isotope variations in the painfully thin remnants of Mars's atmosphere for clues to the timing and nature of this disaster. The key question is: did life get a chance to make its appearance before catastrophe struck?

For Meyer, this issue is of critical importance. "We know life appeared on Earth. But we do not know how easy that process was. It could have been straightforward or it could have been a highly fraught business filled with all sorts of unlikely contingencies."

Mars provides the perfect place to solve that mystery. If its ancient watery, organic soils eventually led to the appearance of primitive living beings before catastrophe struck, we can conclude that life could be relatively commonplace in the cosmos, he argues. "Life appearing separately on two neighbouring worlds would suggest it is a straightforward phenomenon."

But if the sands of Mars turn out never to have supported life, despite their initially attractive properties, life will look a far less likely outcome in the universe. "From that perspective, life on Earth – including humans – may turn out to be a cosmic improbability," adds Meyer. Finding out which version is the right one underscores the importance of Curiosity and future rover missions, which will be designed specifically to answer such questions.

There is more, adds Meyer. Our knowledge about life's appearance on our own planet is abysmal. Billions of years of biological, chemical, meteorological and geological activity on Earth have obliterated all evidence of its origins. "We do not know where it started, how it started, when it started, or what biochemical precursors led to its appearance," Meyer points out.

By contrast, on Mars, life may have flourished for only a very brief period before being extinguished so that signs of its existence may well have been preserved in the planet's dead dust. Their discovery would be like finding biological snapshots frozen in time. "If we can find evidence of organisms' first appearance on Mars, we will be provided with a cookbook for the ingredients of life itself," says Meyer.

The prospects of future scientific excitement emanating from Mars certainly look good, both from Curiosity and from the robot rovers that are scheduled to follow in its wheel-tracks. But for all the wonderful science that is being done there, it is simply the presence of Curiosity, with its sophisticated cameras and detectors, on the planet that provides the greatest satisfaction for those involved in its operations. "I head home after a shift working with Curiosity and sometimes see Mars in the night sky," says Jandura. "The next day I am back at JPL looking at images taken of the Martian surface that have just been sent back by Curiosity. I can see the planet from the rover's perspective and it is utterly thrilling to know that it is there."

Meanwhile back on Earth…Curiosity's successor takes shape in Hertfordshire

ExoMars rover

In a nondescript shed on a minor road near Stevenage railway station, researchers have built the ultimate, out-of-this-world facility. Sand has been spread over the floor of a large hall; red, sandstone boulders have been littered on top; and on the surrounding walls, photographs of Mars's surface have been pasted together to form a complete background landscape. Stand in the middle and you could be on the Red Planet – were it not for the breathable atmosphere and substantial gravity.

However, the real attraction of this alien experience is not its extraterrestrial decoration but a six-wheeled vehicle whose binocular cameras, perched on a five-foot-high central control rod, gives it more than a passing resemblance to Wall-E, the eponymous robot star of the Pixar cartoon. This is ExoMars, the European robot rover scheduled to succeed Curiosity in 2018. The two craft have similar designs, a metal skeleton frame fitted with six wire-rimmed wheels. However, ExoMars possesses one crucial difference: it will be able to drill two metres down into the Martian soil. Curiosity can penetrate only a few centimetres.

"Mars is battered by intense ultraviolet radiation," says Ben Boyes, Deputy Engineering Manager on the ExoMars Rover project. "The prospect of finding life – even very primitive forms of life – on its surface is therefore unlikely. But deeper down, things might be very different."

Due to launch constraints, the ExoMars rover (pictured) being built at Stevenage by the European aerospace company Astrium is much lighter than Curiosity, however. The former will weigh 300kg, a third of Curiosity's weight. As a result, thin solar panels will have to be used to provide power, in contrast to the heavy nuclear generator fitted on Curiosity, a lack of power that puts pressure on ExoMars's designers.

"At night, temperatures on Mars can drop to -120C," says Boyes. "At those temperatures, the craft's electronics can suffer irreparable damage so we have to find clever ways to keep them warm. We will need to charge up a battery to provide night heating and make sure key electronics are insulated – while not adding to the rover's weight."

All these different designs will be tested in Astrium's Mars Yard. "We have to get everything right," adds Boyes. "That includes the sand. Martian sand is very, very fine and can get into the tiniest of gaps. We have be very careful."

ExoMars was originally scheduled to be a European-US collaboration until Nasa, reeling from budget cuts imposed by the White House, pulled out of the mission. At the last minute, rescue came from the Russians, who offered to launch the probe and share its development costs.

The relief experienced by Europe's scientists has been restrained, however – for Russia has a truly dismal record when it comes to Mars missions. A total of 19 have been attempted by Russia (and the Soviet Union before it). Only two partial successes have resulted. The rest have been complete failures. As one jaundiced UK space scientist describes Russia's last-minute intervention: "It's like being rescued while adrift at sea – only to find you have been picked up by the Titanic."

Boyes was more diplomatic. "It's a worry," he admits. "But then any mission to Mars is a worry. Hopefully we will get the success we feel we deserve."

• This article was amended on 31 July 2013. The original incorrectly named the people in the picture at the top of the article as Keith Comeaux and Martin Greco. This has been corrected.